1. Field of the Invention
The present invention generally relates to tunable capacitors, and more particularly, to a tunable capacitor using MEMS (microelectromechanical system) techniques and a method of fabricating such a capacitor.
The tunable capacitor is a key component in electrical circuits such as a variable frequency oscillator (VCO), a tunable amplifier, a phase shifter and an impedance matching circuit. Recently, the tunable capacitor has been increasingly applied to cellular phones.
As compared to a varactor diode, which is a kind of tunable capacitors currently used, the MEMS tunable capacitor has advantages of a small loss and a high Q value. Therefore, there has been considerable activity in the development of practical MEMS tunable capacitors.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a tunable capacitor that is described in Jae Y. Park et al., “MICROMACHINED RF MEMS TUNABLE CAPACITORS USING PIEZOELECTRIC ACTUATORS”, IEEE International Microwave Symposium, 2002.
This tunable capacitor includes a movable electrode substrate 11 and a stationary electrode substrate 15. The movable electrode substrate 11 is made up of a unimorph type of piezoelectric actuators 12 and a movable electrode 13. A stationary electrode 16 is provided on the stationary electrode substrate 15. The stationary electrode substrate 11 and the stationary electrode substrate 15 are bonded by solder bumps 14. By driving the piezoelectric actuators 12, the distance of the movable electrode 13 and the stationary electrode 16 is changed, so that the capacitance formed therebetween can be varied.
FIGS. 2A and 2B are cross-sectional views of a tunable capacitor described in Charles L. Goldsmith et al., “RF MEMS Variable Capacitors for Tunable Filters”, Wiley RF Microwave Computer Aided Design, 1999, pp. 362-374.
Referring to FIG. 2A, a stationary electrode 20 is provided on an insulation layer on a substrate 17. A dielectric layer 19 covers the stationary electrode 20. Spacers 18 are provided on the insulation film. A membrane movable electrode 21 is supported by the spacers 18 so as to face the stationary electrode 20 and the dielectric layer 19. An electrostatic attraction develops between the movable electrode 21 and the stationary electrode 20 across which a dc voltage is applied. The electrostatic attraction brings the membrane movable electrode 21 into contact with the dielectric layer 19. The electrostatic attraction F that acts to reduce the gap between the parallel plates is expressed:   F  =            S              2        ⁢                  d          2                      ⁢          ɛ      0        ⁢          ɛ      r        ⁢          V      2      where S is the area of the plates, d is the distance between the plates, ∈0 is the dielectric constant in vacuum, ∈r is the relative dielectric constant between the plates, and V is the voltage applied across the plates. In a case where a dielectric layer is interposed between the plates, the following relational expression stands between the relative dielectric constant ∈r and the distance d:       d          ɛ      r        =                    d        air                    ɛ        air              +                  d        dielectric                    ɛ        dielectric            where ∈dielectric and ∈air are respectively the relative dielectric constants of the dielectric and air layers, ddielectric and dair are respectively the thicknesses of the dielectric and air layers.
However, the conventional tunable capacitor shown in FIG. 1 has the following disadvantages. There is difficulty in reducing the gap between the movable electrode 13 and the stationary electrode 16 because the gap is defined by the solder bumps 14. This brings about a small initial capacitance of the piezoelectric actuators 12. It is conceivable to increase the degree of distortion of the piezoelectric actuators 12 in order to increase the electrostatic capacitance. However, this attempt reduces the spring performance of the piezoelectric actuators 12. If an external shock is applied to the tunable capacitor at the time of mounting it to an electronic apparatus such as a cellular phone, the movable electrode 13 may be brought into contact with the stationary electrode 16, so that the electrodes 13 and 16 are short-circuited and broken. Thus, the movable electrode 13 cannot be tuned so as to be close to the stationary electrode 16 even by the distortion of the piezoelectric actuators 12, so that a desirable capacitance cannot be obtained.
The conventional tunable capacitance shown in FIGS. 2A and 2B has the following disadvantages. In the case where the dielectric layer is interposed between the parallel plates, a large capacitance can be obtained due to the function of the intervening dielectric layer. However, the relative dielectric constant ∈r changes as the distance d changes. Thus, it is difficult to control the distance between the parallel plates. It follows that the movable electrode 21 and the dielectric layer 19 can assume only two states, namely, the distant state and the contact state. That is, the tunable capacitor has only two capacitance values. In some cases, multiple tunable capacitors having relatively small capacitance values are connected in parallel in order to secure the target capacitance. However, the wiring or interconnection resistance for connecting the multiple tunable capacitors increases, this reducing the Q value (which indicates the insertion loss, and increases as loss deceases).